In the FedEx error logs I'm seeing some responses:
<Notifications xmlns="http://fedex.com/ws/rate/v24">
<Severity>NOTE</Severity>
<Source>crs</Source>
<Code>441</Code>
<Message>This shipment met Shipment Weight Minimum criteria.</Message>
<LocalizedMessage>This shipment met Shipment Weight Minimum criteria.</LocalizedMessage>
</Notifications>

According to their documentation the NOTE response should be treated as SUCCESS but AC isn't showing the rates.
http://images.fedex.com/...ices-FINAL-1-27-2016.pdf

The notifications are:
 Success: Your request was processed
successfully.
 Note: Your request was processed successfully.
The code/message contains additional information
about how your request was fulfilled; you do not
need to take any special action.
 Warning: Your request was processed
successfully. The code/message explains what
was done to fulfill the request; however, some
assumptions may have been made to fulfill the
request and you should determine if the reply
matches intent. A warning message may also be a
notification of a future change